Modified BACE

ABSTRACT

The present invention relates to recombinant human BACE polypeptides. More particularly, the invention relates to recombinant human BACE polypeptides that have a modified amino acid sequence at position 33 of the BACE sequence, as well as to polynucleotides, expression vectors, host cells, and methods for producing the modified recombinant human BACE polypeptides.

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/358,651, filed Feb. 21, 2002.

FIELD OF THE INVENTION

The invention is related to a recombinant human BACE. More particularly, the invention is related to an active human BACE having a modification at amino acid position 33.

BACKGROUND OF THE INVENTION

Alzheimer's disease (AD) is a progressive degenerative disease of the brain primarily associated with aging. Clinical presentation of AD is characterized by loss of memory, cognition, reasoning, judgment, and orientation. As the disease progresses, motor, sensory, and linguistic abilities are also affected until there is global impairment of multiple cognitive functions. These cognitive losses occur gradually, but typically lead to severe impairment and eventual death in the range of four to twelve years.

Alzheimer's disease is characterized by two major pathologic observations in the brain: neurofibrillary tangles and beta amyloid (or neuritic) plaques, comprised predominantly of an aggregate of a peptide fragment know as A beta. Individuals with AD exhibit characteristic beta-amyloid deposits in the brain (beta amyloid plaques) and in cerebral blood vessels (beta amyloid angiopathy) as well as neurofibrillary tangles. Neurofibrillary tangles occur not only in Alzheimer's disease but also in other dementia-inducing disorders. On autopsy, large numbers of these lesions are generally found in areas of the human brain important for memory and cognition.

Smaller numbers of these lesions in a more restricted anatomical distribution are found in the brains of most aged humans who do not have clinical AD. Amyloidogenic plaques and vascular amyloid angiopathy also characterize the brains of individuals with Trisomy 21 (Down's Syndrome), Hereditary Cerebral Hemorrhage with Amyloidosis of the Dutch-Type (HCHWA-D), and other neurogenerative disorders. Beta-amyloid is a defining feature of AD, now believed to be a causative precursor or factor in the development of the disease. Deposition of A beta in areas of the brain responsible for cognitive activities is a major factor in the development of AD. Beta-amyloid plaques are predominantly composed of amyloid beta peptide (A beta, also sometimes designated betaA4). A beta peptide is derived by proteolysis of the amyloid precursor protein (APP) and is comprised of 39-42 amino acids. Several proteases called secretases are involved in the processing of APP.

Cleavage of APP at the N-terminus of the A beta peptide by beta-secretase and at the C-terminus by one or more gamma-secretases constitutes the beta-amyloidogenic pathway, i.e. the pathway by which A beta is formed. Cleavage of APP by alpha-secretase produces alpha-sAPP, a secreted form of APP that does not result in beta-amyloid plaque formation. This alternate pathway precludes the formation of A beta peptide. A description of the proteolytic processing fragments of APP is found, for example, in U.S. Pat. Nos. 5,441,870; 5,721,130; and 5,942,400.

An aspartyl protease has been identified as the enzyme responsible for processing of APP at the beta-secretase cleavage site. The beta-secretase enzyme has been disclosed using varied nomenclature, including BACE, Asp, and Memapsin. See, for example, Sinha et. al., 1999, Nature 402:537-554 (p501) and published PCT application WO00/17369.

Several lines of evidence indicate that progressive cerebral deposition of beta-amyloid peptide (A beta) plays a seminal role in the pathogenesis of AD and can precede cognitive symptoms by years or decades. See, for example, Selkoe, 1991, Neuron 6:487. Release of A beta from neuronal cells grown in culture and the presence of A beta in cerebrospinal fluid (CSF) of both normal individuals and AD patients has been demonstrated. See, for example, Seubert et al., 1992, Nature 359:325-327.

It has been proposed that A beta peptide accumulates as a result of APP processing by beta-secretase, thus inhibition of this enzyme's activity is desirable for the treatment of AD. In vivo processing of APP at the beta-secretase cleavage site is thought to be a rate-limiting step in A beta production, and is thus a therapeutic target for the treatment of AD. See for example, Sabbagh, M., et al., 1997, Alz. Dis. Rev. 3, 1-19.

Published international patent applications WO 00/47618, WO 01/23533 and WO 00/17369 identify the beta-secretase enzyme and various methods of its use. To better understand the mechanism of action of β-secretase and help explore novel strategies for drug discovery for Alzheimer's disease, it has become important to elucidate the 3-dimensional structure of its zymogen. From the 3-dimensional structure, it has been possible to explore possible mutations in BACE which will inhibit enzyme activity as well as explore potential active site for target molecules.

SUMMARY OF THE INVENTION

The present invention relates to an isolated polypeptide sequence comprising human BACE having the modification Pro33Lys. The polypeptide can comprise at least a portion of the transmembrane domain, at least a portion of the C-terminal tail, and/or at least a portion of the signal peptide.

The invention also relates to a composition comprising an active human BACE enzyme comprising the pro-enzyme sequence of BACE having the modification Pro33Lys. The polypeptide can comprise at least a portion of the transmembrane domain, at least a portion of the c-terminal tail, and/or at least a portion of the signal peptide.

Further, the invention relates to an isolated polypeptide of SEQ ID NO: 2.

The invention relates to an isolated polynucleotide comprising a polynucleotide sequence which, by virtue of the degeneracy of the genetic code, encodes P33K-BACE. The P33K-BACE can have the sequence of SEQ ID NO: 2. The isolated polynucleotide can comprise the nucleotide sequence of nucleotides 70-1365 of SEQ ID NO: 8.

The invention also relates to an expression vector comprising the polynucleotide sequence encoding P33K-BACE. The expression vector produces a P33K-BACE polypeptide when said expression vector is present in a compatible host cell. The expression vector can comprise the polypeptide sequence of SEQ ID NO: 2.

A recombinant host cell comprising the expression vector having the polynucleotide sequence encoding P33K-BACE.

A method for producing a P33K-BACE polypeptide comprising culturing the recombinant host cell having an expression vector encoding P33K-BACE under conditions sufficient for the production of said polypeptide and recovering the polypeptide from the culture. The host cell can be E. Coli.

A method of producing active P33K-BACE comprising recovering the P33K-BACE from the culture of host cells according and diluting the polypeptide 20-50 fold with water having a temperature of about 1 to 15° C.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the amino acid sequence of human BACE [SEQ ID NO: 1]

FIG. 2 shows the amino acid sequence of an embodiment of the P33K-BACE polypeptide [SEQ ID NO: 2].

FIG. 3 shows a sequence alignment of β-secretase zymogen (pbsz) [SEQ ID NO: 3], β-secretase (1 fkn) [SEQ ID NO: 4], progastricsin (1 htr) [SEQ ID NO: 5] and pepsinogen (3 psg) [SEQ ID NO: 6]. The lines indicate the residue pair involved in forming disulfide bond as observed in 1 fkn. The codes representing the conserved residues at the active site for the aspartyl protease family are residues 93-95 and residues 289-291 for pbsz, residues 91-93 and residues 274-276 for 3 psg, and residues 91-93 and residues 276-278 for 1 htr. The signal peptide segments (residues 1-21 for pbsz, residues 1-16 for 1 htr, and residues 1-15 for 3 psg) were not included for the alignment operation because they will be cleaved off by signal peptidase during the secretory process.

FIGS. 4A and 4B are the DNA and predicted amino acid sequence of the modified recombinant BACE expressed from a pET11a-P33K-BACE construct: amino acid sequence [SEQ ID NO: 7]; DNA sequence [SEQ ID NO: 8].

DETAILED DESCRIPTION

The invention provides for a human BACE polypeptide having a mutation at amino acid position 33 (position 54 if the leader sequence is counted). “BACE” (beta-site APP-cleaving enzyme), refers to an enzyme that mediates cleavage at the beta-site of APP. This enzyme is also known as beta-secretase, Asp2, and Memapsin 2. BACE has been described, for example, in WO 00/17369, WO 00/47618 and WO 01/23533, each of which is incorporated herein by reference in their entirety. BACE comprises an aspartyl protease and contains the classical consensus aspartyl protease active site motif (DTG/DSG).

Features of the human BACE polypeptide shown in FIG. 1 include a 21 amino acid leader (signal or pre-) sequence shown in italics, and a 24 amino acid pro-sequence, shown in bold type. T¹ marks the start of the pro-sequence. A 27 amino acid transmembrane domain is underlined, and is followed by the cytosolic C-terminal tail. Disulphide bridges are formed by cysteines (Cys¹⁹⁵-Cys³⁹⁹, Cys²⁵⁷-Cys⁴²²; and Cys³⁰⁹-Cys³⁵⁹). For the purposes of this invention BACE may optionally include (1) the complete, or a portion of, the signal sequence at the N-terminus, (2) the complete, or a portion of, the transmembrane domain, and/or (3) the complete transmembrane domain with the complete, or a portion of, the C-terminal tail. “A portion of” refers to any number of amino acids in the various sequences.

“Pro33Lys-BACE,” or “P33K-BACE”, refers to the enzyme including the polypeptide sequence of recombinant human BACE having a proline to lysine mutation at position 33 as shown in FIG. 2. Accordingly, when appropriately refolded, recombinant P33K-BACE is an active BACE enzyme including a peptide sequence of at least amino acids 1-432 of BACE with the P33K mutation. Pro33Lys-BACE and P33K-BACE refer to the polypeptide as it may optionally include (1) the complete, or a portion of, the signal sequence at the N-terminus, (2) the complete, or a portion of, the transmembrane domain, and/or (3) the complete transmembrane domain with the complete, or a portion of, the C-terminal tail. “A portion of” refers to any number of amino acids in the various sequences.

The “beta secretase zymogen” or the “BACE zymogen” refers to the BACE which includes the 24 amino acid pro-sequence. Generally, an enzyme will be inactivated by the existence of its pro-peptide. However, unlike most other zymogens, the existence of the pro-peptide does not seem to have a significant impact on the activity of BACE. To understand the effect of the pro-sequence in BACE, the three dimensional structure of BACE was elucidated. From the three dimensional structure, it has been possible to explore possible mutations in BACE which will inhibit enzyme activity.

As a first step in the elucidation of the three-dimensional structure of the β-secretase zymogen, a sequence alignment was performed for the β-secretase zymogen as disclosed by Vassar et al., 1999, Science 286:735-741 (abbreviated as pbsz) [SEQ ID NO: 3], β-secretase (1 fkn) [SEQ ID NO: 4], progastricsin (1 htr) [SEQ ID NO: 5], and pepsinogen (3 psg) [SEQ ID NO: 6] was performed using the PILEUP program in the GCG package (Genetic Computer Group, Madison, Wis.). Pepsinogen and progastricsin are pro-enzymes in the family of aspartyl proteases that includes β-secretase. The aligned result is given in FIG. 3, where the pro-peptides are underlined. For the case of pbsz, the signal peptide includes residues 1-21 (not shown), the pro-peptide includes residues 22-45 (Bennett, et al, 2000, Journal of Biological Chemistry 275:37712-37717), and the main-chain includes residues 46-446 (with the active site aspartates at 93 and 289). The transmembrane and intracellular domains that follow residue 446 are outside the scope of the study.

The numbering of amino acids in sequence pbsz of FIG. 3, and the numbering of the corresponding amino acids in FIGS. 1 and 2 differ since the numbering of FIG. 3 assumes the 21 amino acid signal sequence that is shown in italics in FIG. 1. Accordingly, position 54 of pbsz in FIG. 3 is same as position 33 in FIGS. 1 and 2. Thus, while P33K-refers to the proline to lysine mutation at position 33 of BACE as shown in FIG. 1, it should be understood that, if referring to pbsz of FIG. 3, the same mutation could be referred to as P54K.

Using the alignment shown in FIG. 3, the 3-D model of the β-secretase zymogen (pbsz) was constructed from (i) the X-ray coordinates of the protease domain of β-secretase (1 fkn.pdb) recently determined by Hong et al., 2000, Science 290:150-153, and (ii) the X-ray coordinates of pepsinogen (3 psg.pdb) determined by Hartsuck et al, 1992, PROTEINS: Structure, Function and Genetics 13:1-25. The pro-segment of the pepsinogen structure provided the basis for a homology model of the β-secretase pro-segment, which was grafted onto the β-secretase protease domain, using the procedure described below.

Since the pro-peptide segment and the protease domain of the model were derived from two different templates, an operation for a smooth connection at a proper site for the two structures was needed. This was performed as follows. The template structure 3 psg.pdb was superimposed onto the template structure 1 fkn.pdb, using the commercial software package, MOE (Chemical Computing Ltd.). During the superimposition process the entire structure of 3 psg underwent a translational and rotational motion, and hence the coordinates of 3 psg changed, although the coordinates of 1 fkn remained unchanged. It was observed from the superimposed pair that, starting from Gly-74 and proceeding in the C-terminal direction (the residue number is counted based on the sequence of pbsz as shown in FIG. 2), the backbone chain of 1 fkn followed almost the same trajectory as that of the backbone chain of 3 psg, for most of the N-terminal lobe of the bilobal structures. Moving in the N-terminal direction from Gly-74 of β-secretase, however, the structures diverged markedly. Accordingly, residue 74 of the β-secretase structure became the joining point for grafting on the pro-segment of pepsinogen (in the form of the actual β-secretase pro-segment sequence). A smooth connection between residues 16-72 of 3 psg and residues 74-446 of 1 fkn (FIG. 2) was realized without causing any structural conflicts. The structure thus obtained was then used as a combined template to develop the final 3-D model of the β-secretase zymogen (pbsz) by the segment matching modeling method. Levitt, M., 1992, J. Mol. Biol. 226:507-533.

The segment matching approach (in the MOE software) employs a database of known protein structures to build an unknown target structure based on an amino acid sequence alignment. In this case the target structure was the β-secretase zymogen, i.e. the pro-segment plus the protease domain of β-secretase. The target structure was first broken into a set of short segments. The database was then searched for matching segments on the basis of amino acid sequence similarity and compatibility with the target structure. The process was repeated 10 times and an average model was generated, followed by energy minimization of the entire pro-enzyme to create the final model. The structure thus obtained uniquely defined the atomic coordinates of not only residues 22-45, the pro-segment of pbsz, but also the segment of residues 46-55 in the main chain that was missing in the crystal structure of 1 fkn.pdb (Hong et al., 2000). Furthermore, although the majority of the protease domain (residues 75-446) of pbsz was almost identical to the corresponding sequence in 1 fkn, a small transition-linking part of the protease domain (residues 56-74) was affected owing to the existence of the pro-segment. This procedure was originally shown to be highly accurate for eight test proteins ranging in size from 46 to 323 residues, where the all-atom root-mean-square deviation (RMSD) of the modeled structures was between 0.93 angstrom and 1.73 angstrom (Levitt, M,. 1992, J. Mol. Biol. 226:507-533). This method was previously used to model the structure of the protease domain of caspase-8, at a time before the X-ray coordinates were released for caspase-3 (13). In that particular study, the atomic coordinates of the catalytic domain of caspase-3 were predicted based on the X-ray structure of caspase-1, and then the caspase-3 structure thus obtained served as a template to model the protease domain of caspase-8. After the X-ray coordinates of caspase-3 protease domain were finally released and the X-ray structure of the caspase-8 protease domain was determined (Watt, et al, 1999, Structure 7:1135-1143), it turned out that the RMSD for all the backbone atoms of the caspase-3 protease domain between the X-ray and predicted structures was 2.7 angstrom, while the corresponding RMSD was 3.1 angstrom for caspase-8, and only 1.2 angstrom for its core structure. This indicates that the computed structures of caspase-3 and −8 were quite close to the corresponding X-ray structures.

Since the origins of the protease domain of the model came from crystallographic coordinates, it was expected that the final energy minimized model of that domain would retain most, if not all, of the experimental attributes, and that was the case. In particular, the model retained the three pairs of disulfide bonds, i.e. Cys²¹⁶-Cys⁴²⁰, Cys²⁷⁸-Cys⁴⁴³, and Cys³³⁰-Cys³⁸⁰. This implies that the existence of the pro-peptide segment would not destroy the disulfide bonds but rather likely facilitate a proper folding for forming the three pairs disulfide bonds as observed in an active protease domain, Haniu, M. et al, 2000, Journal of Biological Chemistry 275:21099-21106. Proceeding in the N-terminal direction from the pro-segment attachment point, the backbone traces a path from one end of the active site cleft, toward the center, then covers over the “flap” of the active site as described in Hong et al., 2000. It then continues toward the far end of the active site, makes a turn, and returns via two helices to near its origination point. Its overall structure is somewhat similar to the pepsinogen pro-segment from which it was derived, but with a key difference described below. An overlay of the β-secretase crystal structure with the pro-enzyme model shows some differences in side chain positioning induced by the presence of the pro-segment, and very minor differences in distal positioning, likely due to the energy minimization.

Inactivation of an enzyme by its pro-peptide is generally thought to be due to physical blockage of the catalytic site, preventing access to substrate. In the case of aspartyl proteases, a pro-segment could also disrupt the catalytically-required water molecule between the two aspartates. A comparison of the 3-D structures of pepsinogen, pro-gastricsin, and the β-secretase pro-enzyme model indicates that the pro-segments of all three cover up the catalytic site, and therefore should block access to substrate. The dynamics of protein motion, however, could allow periodic unfolding of the pro-segments exposing the catalytic clefts to enable substrate processing. Yet only for the β-secretase pro-enzyme is substrate processing known to occur, so there is something unique about the positioning of its pro-segment.

As mentioned above, the substrate amide bond hydrolysis by aspartyl proteases requires the participation of a water molecule (Silverman, R. B., 2000, The Organic Chemistry of Enzyme-Catalyzed Reactions, Chapter 2, Academic Press, San Diego). The catalytic reaction involves (i) the β-carboxyl groups of the two Asp residues (i.e., Asp-93 and Asp-289 for the case of β-secretase) at the active site being brought in to close proximity to activate a water molecule by forming hydrogen bonds with it; (ii) the nucleophilic attack of the activated water molecule on the carbonyl carbon atom of the scissile peptide bond to form the tetrahedral intermediate; (iii) the decomposition of the tetrahedral intermediate to yield the product of cleaved peptides and active enzyme. Accordingly, before a peptide bond is cleaved by an aspartyl protease, the two Asp residues at the active site must first activate a water molecule by forming four hydrogen bonds with it.

However, for the case of pepsinogen (3 psg), the two active site Asp residues, i.e., Asp-91 and Asp-274 (FIG. 2), have already formed bonds to Lys-51 of the pro-peptide by two salt bridges: one is between O^(δ1) of Asp-91 and N^(ζ) of Lys-51, and the other between O^(δ2) of Asp-274 and N^(ζ) of Lys-51, as clearly shown in the X-ray structure determined by Hartsuck et al. As is well known, salt-bridges are stronger than hydrogen bonds. This will certainly disrupt the two active site Asp residues in activating a water molecule, and hence the activity of the pepsinogen in cleaving a peptide bond is impeded by the existence of the pro-peptide segment. A similar situation also occurs in the case of progastricsin (1 htr), where the two active site Asp residues, i.e. Asp-91 and Asp-276, have also formed two salt bridges with Lys-53 of the pro-peptide: one is between O^(δ1) of Asp-91 and N^(ζ) of Lys-53, and the other between O^(δ2) of Asp-276 and N^(ζ) of Lys-53, as shown by the X-ray structure determined by Ivanov et al, 1990, Biochim. Biophys. Acta, 1040:308-310. Accordingly, one could view the salt bridges to the aspartates as a “locking” mechanism that holds the pro-segment in place and prevents the proper positioning of a catalytic water molecule.

The microenvironment is much different in the β-secretase zymogen model, where no salt bridges are observed between the pro-peptide segment and the two active site Asp residues, i.e., Asp-93 and Asp-289 (FIG. 2). According to the model, it is Pro-54 that corresponds to the Lys locations in the other two pro-enzyme structures from both sequence alignment (FIG. 2) and 3-D structure. Because the numbering of the BACE (pbsz) in FIG. 2 assumes a 21 amino acid signal sequence, Pro-54 in FIG. 2 is the same residue as Pro-33 in FIG. 1. However, a proline side-chain cannot form a salt bridge. Thus, for the case of the β-secretase zymogen, the “locking” mechanism is absent and there is no pro-segment side-chain in the location of the catalytic water position.

According to this model, therefore, the existence of the pro-peptide segment should not completely reduce the activity of β-secretase. This structural observation is supported by the recent experimental observations from the following two independent groups. Shi et al., 2001, J. Biol. Chem. 276:10366-10373 observed that, when assayed with a polypeptide substrate, the k(cat)/K(m) of β-secretase with the pro-segment intact is only 2.3-fold less than β-secretase. They concluded that the pro-domain of β-secretase “does not suppress activity as in a strict zymogen but does appear to facilitate proper folding of an active protease domain.” Benjannet et al., (2001 J. Biol. Chem. 276:10879-10887), observed that “pro-BACE can produce significant quantities of Swedish mutant βAPP_(SW) β-secretase product C99,” and hence the pro-domain has little effect on the BACE active site.

While the absence of the “locking mechanism” in the pro-BACE model provides a possible explanation for the unusual retained activity of the pro-enzyme, the design of experiments to test the hypothesis is complicated by the fact that Pro54 (FIG. 3), or Pro33 (FIG. 1), in the pro-BACE model imparts a substantially different backbone trajectory in the region of that residue, as compared to what is observed in the two comparator crystal structures (3 psg and 1 htr). This observation would be expected, due to the cyclic conformational constraints of a proline residue. Mutation of the proline to a lysine in pro-BACE would, correspondingly, also be expected to change the backbone characteristics in that region.

As shown in the following experiments, the P33K-BACE has essentially the same activity of BACE. This suggests that confirmation of the hypothesis by experimental modification of the BACE pro-segment would need to involve more than just the Pro33Lys mutation, to include one or more additional residues that would enable the nearby pro-segment backbone to more closely mimic those of the comparator pro-enzymes.

Recombinant BACE, including recombinant P33K-BACE, can be produced, for example, in E. coli or other suitable host cells, by expressing a construct that contains at least a portion of a cDNA encoding P33K-BACE, for example, encoding at least a portion of the amino acid sequence shown in FIG. 2. The construct can also contain additional nucleotide sequences that may, for example, assist in purification or expression of the recombinant polypeptide, as desired.

The polynucleotide construct for expressing P33K-BACE may include nucleotides coding for the signal peptide, the transmembrane domain and/or the c-terminal tail or portions thereof. Such constructs may be assembled using routine methods by those skilled in the art. The complete polynucleotide sequence of BACE may be found, for example in Vassar et al, Science 286:7353-741 (1999) and the PCT publications that have been incorporated by reference herein. In addition, GenBank Accesion No. NM 012104 describes a number of known alleles of the BACE sequence. In addition, “silent” nucleotides substitutions may be introduced into the BACE construct sequence to enable better expression of the sequence in a desired organism, or for other reasons. Accordingly, due to the degeneracy of the genetic code, the polypeptide sequence of BACE may be expressed from a vast number of polynucleotide sequences. The present invention is directed to any polynucleotide sequence encoding P33K-BACE.

When expressed in E. coli, recombinant P33K-BACE accumulates intracellularly in an insoluble form, resulting in phase-bright inclusions in the cytoplasm (inclusion bodies). The protein in the inclusion bodies can be a mixture of monomeric and multimeric forms of the protein, both reduced and oxidized.

Processes designed to recover biologically active, soluble protein from the insoluble cellular material generally include the steps of: (1) cell lysis, (2) isolation of inclusion bodies, (3) solubilization of protein from inclusion bodies, (4) refolding of solubilized protein, and (5) purification of the active protein. Each of these steps will be described in relation to the invention below.

Useful constructs for the production of P33K-BACE are designed to express a selected portion of the P33K-BACE polypeptide. The polynucleotide encoding the P33K-BACE polypeptide can be operably linked to suitable transcriptional or translational regulatory sequences in an expression construct. Regulatory sequences include transcriptional promoters, operators, enhancers, mRNA ribosomal binding sites, and other sequences that control transcription or translation. Nucleotide sequences are “operably linked” when the regulatory sequence functionally relates to the polynucleotide encoding P33K-BACE. Thus, a promoter nucleotide sequence is operably linked to a polynucleotide encoding P33K-BACE if the promoter nucleotide sequence directs the transcription of the P33K-BACE sequence.

The polynucleotide is cloned into appropriate expression vectors for expression in E. coli. Generally, an expression vector will include a selectable marker and an origin of replication, for propagation in E. coli. Expression vectors generally comprise one or more phenotypic selectable marker genes. Such genes generally encode, for example, a protein that confers antibiotic resistance or that supplies an auxotrophic requirement.

A polynucleotide can encode a P33K-BACE polypeptide having an N-terminal methionine to facilitate expression of the recombinant polypeptide in a prokaryotic host, for example, for expression in E. coli. The N-terminal methionine can optionally be cleaved from the expressed P33K-BACE polypeptide. The polynucleotide can also encode other N-terminal amino acids added to the P33K-BACE polypeptide that facilitate expression in E. coli. Such amino acids include, but are not limited to, a T7 leader sequence, a T7-caspase 8 leader sequence, and known tags for purification such as the T7-Tag MASMTGGQQMGR [SEQ ID NO: 9] that allows binding of antibodies, or a six-histidine tag (His)₆ that allows purification by binding to nickel. Other useful peptide tags include the thioredoxin tag, hemaglutinin tag, and GST tag. These and other amino acid tags can be encoded by polynucleotides added to either terminus of the polynucleotide encoding P33K-BACE.

The polynucleotide of the expression construct can encode a P33K-BACE polypeptide that is truncated by removal of all or a portion of the C-terminal cytoplasmic tail, the transmembrane domain, the membrane proximal region, or any combination of these. The expression constructs can also encode cleavage sites for selected enzymes, to improve purification of the expressed protein or to assist in expression of the enzyme, when desired.

It has been found that active recombinant BACE protein can terminate at S⁴³², lacking the transmembrane domain and cytosolic tail region. This provides BACE in a soluble form, that is, a form that is not membrane-bound. Accordingly, in the following examples, P33K-BACE was terminated at S⁴³² to compare activity with the known active recombinant BACE.

For efficient expression, one or more codon of the polynucleotide sequence encoding P33K-BACE can be modified, using such techniques as site directed mutagenesis, to eliminate GC-rich regions of strong secondary structure known to interfere with efficient cloning or expression of the recombinant protein. Codons can also be optimized for expression in E. coli, for example, according to published codon preferences. Underlined nucleotides in FIG. 4A show preferred codon changes.

An expression construct containing a polynucleotide encoding P33K-BACE can be used to transform bacteria, for example E. coli, in order to produce P33K-BACE protein. Production of the protein can be inducible or constitutive, depending upon the control elements provided in the vectors. For example, expression constructs are transfected into a bacterial host, such as E. coli BL21 codon plus (DE3) RP (Stratagene) and grown in suitable media, such as Luria broth supplemented with 100 micrograms/ml ampicillin and 34 micrograms/ml chloromphenicol. When cells have grown to a desired density, in general, when the absorbance of the culture at 550 nm is between 0.5 and 0.6, expression is induced. For example, the T7 or T5 lac promoter promotes expression of the operably linked P33JK-BACE polynucleotide upon addition of IPTG (for example, to a final concentration of about 1 mM) to the culture media. After induction, for example, about three hours, the cell pellet is collected and can be stored, generally at −70° C., for later enzyme purification.

The expressed recombinant enzyme accumulates intracellularly in an insoluble form, as inclusion bodies. To recover the enzyme from insoluble cellular material, bacterial cells are pelleted from the bacterial cell culture, lysed, and the inclusion bodies are isolated from the lysed cells. The recombinant enzyme can then be isolated from the isolated inclusion bodies.

Generally, lysing of cells to obtain the protein inclusion bodies can be accomplished using a number of known methods, including mechanical and chemical techniques. Sonication and freeze-thaw techniques are generally not practical for the volume of cells being disrupted. However, any commercially available device that uses a pressure differential to disrupt the cells, such as a French Press or a Rannie apparatus, is acceptable, assuming the overall handling capacity is similar or greater than these instruments. Detergent solubilization is not generally a practical solution, since removal of the detergent can pose a difficult challenge and may influence subsequent refolding efforts. Detergents may solubilize contaminating proteins and nucleic acids together with some or all of the protein of interest from the inclusion bodies, and thus is not a desirable option. Once the cells have been lysed, the inclusion bodies may be washed to remove protein contaminants associated with or entrapped in the inclusion bodies. If not removed, such contaminants could interfere with or prevent refolding of the enzyme.

For example, to obtain inclusion bodies, bacterial cells can be suspended in a suitable buffer that may contain a salt such as sodium chloride, a chelating agent such as EDTA, or both. Suspended cells are then lysed using, for example, a French Press or a Rannie apparatus. The insoluble cellular material obtained is washed in buffer and can be stored and frozen at −20° C. overnight.

Protein aggregates (inclusion bodies) are solubilized and then refolded to obtain active protein. Reagents that can be used to solubilize P33K-BACE include urea, guanidine HCl, guanidine thiocyanate, and the like, generally at a concentration of about 6 to 8M. Reducing agents, such as beta-mercaptoethanol (BME), glutathione (gamma-Glu-Cys-Gly; or GSH, Sigma Cat. No. G-6529); or DTT (dithiothreitol, Sigma Cat. No. D-0632), and the like can also be used. These reducing agents can be used separately or in combination to provide the isolated protein in a reduced form (random coil). These agents can reduce the presence of dimers and higher molecular weight multimers, as well as reduce improper folding, for example, as a result of cysteine residues within the protein, or reduce aggregation of the protein.

Solubilization of P33K-BACE present in inclusion bodies can be achieved via treatment with a solubilizing agent at a high pH (about pH 10-11), and in the presence of a reducing agent such as BME. For example, the insoluble cellular material can be solubilized in 8 M urea, 1 mM EDTA, 1 mM glycine, 100 mM Tris base (pH 10.1-10.6), and 100 mM BME. An aliquot of sample is then diluted, for example, 10-fold, centrifuged, and the optical density (OD) at 280 nm is measured. Sample is diluted to adjust the OD to about 5.0 and pH to approximately 10.1. The sample is then diluted in 8 M urea buffer without a reducing agent. Thereafter, the reducing agent, for example, BME, is added to make the total molarity of the reducing agent about 10 mM. Dithiothreitol (DTT), reduced glutathione (GSH) and oxidized glutathione (GSSH) are added to the solution to obtain final concentrations of 10 mM DTT, 1 mM GSH, and 0.1 mM GSSG, and the pH of the solution is adjusted to 10.3-10.5. This procedure provides P33K-BACE in reduced form.

Alternatively, insoluble cellular material can be solubilized and the enzyme provided in reduced form by washing in 10 mM Tris buffer (pH 8), 1 mM EDTA (TE). Inclusion bodies are then extracted with 8 M urea, 100 mM AMPSO (pH 10.5-10.8), 1 mM glycine, 1 mM EDTA, and 100 mM BME. AMPSO is 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (Sigma Cat. No. A1911). After centrifugation, the protein concentration of the supernatant can be adjusted by dilution with buffer to approximately 5.0 at A₂₈₀. The protein is then diluted with 8 M urea, 100 mM AMPSO, 1 mM glycine, 1 mM EDTA, and BME at an adjusted concentration of 10 mM. Other buffer solutions can be substituted for AMPSO, such as CAPS or Tris. CAPS is (3-[cyclohexylamino]-1-propanesulfonic acid, Sigma Cat. No. C-2632).

Once the protein has been solubilized, it can be refolded into the correct conformation to provide active enzyme. Typically, refolding of an expressed recombinant enzyme can be accomplished by removing the solubilizing agent and replacing it with an aqueous buffer, for example, by dialysis or dilution. Generally, for proteins with disulfide bridges, oxidation of the reduced protein occurs prior to or concomitant with refolding.

According to the invention, reduced protein P33K-BACE is refolded by considerably diluting (20 to 50 fold, generally 20 to 30 fold) the enzyme in a cold, aqueous solution such as water, optionally to a final concentration of about 10 μg to 30 μg P33K-BACE per ml of solution. Water is preferred, generally at a temperature of about 4° C. to 15° C.

Generally, refolding of recombinantly expressed P33K-BACE is accomplished by permitting the diluted enzyme solution (at about pH 10-11) to rest at about 4° C.-15° C. in, for example, a coldroom or refrigerator for approximately 3-5 days.

For example, as shown in the Examples below, solubilized, recombinant P33K-BACE can be diluted in water (20-25 fold), optionally to a final concentration of approximately 10 micrograms to 30 micrograms P33K-BACE per ml of water, and generally at a pH of about 10.5-10.8. This mixture is maintained at temperatures of approximately 4° C. to approximately 15° C. for several days (3-5) and assayed periodically for enzymatic activity. Activity assays can be performed at this resting stage, starting at about 20 to 24 hours after the initial dilution step.

The refolded enzyme can be purified using standard liquid chromatography techniques, such as, for example, cation or anion exchange chromatography (available, for example, from Amersham Pharmacia Biotech), hydrophobic interaction (available, for example, from Toso Haas), dye interaction (available, for example from Sigma), ceramic hydroxyapatite (available, for example, for Bio-Rad), affinity chromatography (for example, using an inhibitor that binds active enzyme), or size exclusion chromatography (for example, Sephacryl-S100 or S200 column purification as well as resins from BioRad, Toso Haas, Sigma, and Amersham Pharmacia Biotech). One or a combination of these purification techniques can be used according to the invention to provide purified, recombinant P33K-BACE. Anion exchange chromatography using, for example, Q-sepharose, Mono-Q, or Resource Q column purification provides useful separation.

Activity of the refolded, purified recombinant P33K-BACE can be determined by incubating the refolded enzyme with a suitable substrate under conditions to allow cleavage of the substrate. The substrate can be labeled with a detectable marker, such as a fluorescent label, to allow detection of cleavage events.

Suitable substrates are peptides that include a P33K-BACE cleavage site. For example, the synthetic peptides (SEISY-EVEFRWKK) (SEQ ID NO: 10) and (GLTNIKTEEISEISY-EVEFRWKK) (SEQ ID NO: 11) can be cleaved by the recombinant P33K-BACE (at the site marked by “-”). Additional substrates suitable for BACE cleavage include the non-limiting examples, (SEVNL-DAEFRWKK) (SEQ ID NO:12) and (GLTNIKTEEISEVNL-DAEFRWKK) (SEQ ID NO:13), containing the APP Swedish Mutation.

The substrate can be labeled with a suitable detectable marker to permit visualization of cleavage. Assays to detect activity of recombinantly produced P33K-BACE can measure retention or liberation of the detectable marker. Suitable detectable markers include, for example, radioactive, enzymatic, chemiluminescent, or fluorescent labels. In some embodiments, the substrate can include internally quenched labels that result in increased detection after cleavage of the substrate. The substrate can be modified to include a paired fluorophore and quencher including, but not limited to, 7-amino-4-methyl coumarin and dinitrophenol, respectively, such that cleavage of the substrate by P33K-BACE results in increased fluorescence as a result of physical separation of the fluorophore and quencher. Other paired fluorophores and quenchers include bodipy-tetramethylrhodamine and QSY-5 (Molecular Probes, Inc.).

In a variant of this embodiment, biotin or another suitable tag can be placed on one end of the peptide to anchor the peptide to a substrate assay plate, and a fluorophore can be placed at the other end of the peptide. Useful fluorophores include those listed herein, as well as Europium labels such as W8044 (EG&G Wallac, Inc.). One exemplary label is Oregon green that can be coupled to a cysteine residue. Cleavage of the substrate by P33K-BACE will release the fluorophore or other tag from the plate, allowing detection of an increase in retained fluorescence.

Further examples of detectable markers include a reporter protein amino acid sequence coupled to the substrate. Exemplary reporter proteins include a fluorescing protein (for example, green fluorescing proteins, luciferase, and the like) or an enzyme that is used to cleave a substrate to produce a colorimetric cleavage product. Also contemplated are tag sequences that are commonly used as epitopes for quantitative assays. Preferably, the detectable markers do not interfere with binding of P33K-BACE to the substrate, or subsequent cleavage of the substrate. For example, detectable markers can be provided in a suitable size that does not interfere with P33K-BACE activity. In some embodiments, detectable markers can be coupled to the substrate using spacers.

EXAMPLES Example 1 Cloning of P33K-BACE

An expression construct for producing recombinant P33K-BACE protein in E. coli was prepared by site-directed mutagenesis from an existing construct referred as pET11a-BACE. This construct contains nucleotides coding for the following sequence: T7 tag (MASMTGGQQMGR)-GSM-BACE (A⁻⁸-S⁴³²), where the expressed BACE fragment is truncated at both N-terminal and C-terminal regions as compared with the sequence shown on FIG. 1. The insert encodes a protein lacking the transmembrane domain and the 13 N-terminal residues of the leader sequence. A methione codon was inserted adjacent to the first BACE codon, to facilitate removal of the BACE insert with BamHI and potential subcloning for expression of the BACE sequence without the T7 tag. However, this methionine residue is not necessary if the T7 tag is included. The BACE cDNA sequence contains preferred codons for expression in E. coli. These codon changes are underlined in FIG. 4A.

pET11a-BACE can be prepared by techniques well known to one skilled in the art. The insert is obtained by PCR from a full length BACE cDNA. The PCR primers are designed to amplify the BACE sequence from Ala⁻⁸ to Ser ⁴³², including BamHI sites for insertion into the pET11a vector (Novagen, Madison, Wis.). Examples of PCR primers that can be used are: [SEQ ID NO: 14] PF1 5′ - GGCA GGA TCC ATG GCG GGA GTG CTG CCT GCC CA (Forward) [SEQ ID NO: 15] PF2 5′ - GGC AGG ATC CTA TGA CTC ATC TGT CTG TGG AAT G (Reverse)

The PCR product is gel-purified, digested with restriction enzymes, and ligated to the corresponding sites of vector pET11a. The vector includes the T7 lac promoter, permitting induced expression on addition of IPTG.

The codon changes (underlined on FIG. 4A) can be introduced by site directed mutagenesis, using oligonucleotide primers and PCR and the method described below for mutagenesis of P33K. These changes are not necessary for successful expression of BACE in E. coli, but may improve yield of the re-foldable protein by eliminating rare codons.

In this embodiment of the invention, the pET11a-BACE construct was derived from two pre-existing clones. One clone, referred to as pET11a-BACE-J, contained the desired codon changes but lacked the two C-terminal cysteines. The other clone referred as pQE80L-BACE (MRGS (H)₆ GS GSIGTD-BACE: T¹-S⁴³²) contains all the required cysteins (6), but lacks Ala⁻⁸ to Gly⁻¹ (pQE80L, Qiagen). Two overlapping cDNA fragments were generated by PCR. BACE-encoding polynucleotides 37 to 880 as shown in FIGS. 4A and 4B were amplified from pET11a-BACE-J, overlapping 20 nucleotides with the remaining BACE sequence, which was amplified from pQE80L-BACE (nucleotides 861-1368 which includes a stop codon not present at this position in BACE).

The PCR primers for BACE-encoding polynucleotides 37 to 880 amplification of pET11a-BACE-J were: [SEQ ID NO: 16] PF3 5′- GGCA GGA TCC ATG GCT GGT GTT CTG CCA GCT [SEQ ID NO: 17] PR4 5′- T GCC ACT GTC CAC AAT GCT C Primer PF3 includes preferred codon changes in addition to those shown in FIG. 4A.

The overlapping segment from pQE80L-BACE including the rest of the c-terminal amino acids was amplified in a separate PCR reaction, using the primers: [SEQ ID NO: 18] PR5 - 5′ GGCAGGATCCTA TGA CTC ATC TGT CTG TGG AAT 3′ (reverse) [SEQ ID NO: 19] PF6 - 5′ G AGC ATT GTG GAC AGT GGC A 3′ (forward).

The PCR conditions were as follows: one initial cycle of denaturation at 95° C., 30 seconds, 30 cycles of 30 seconds denaturation at 95° C., 30 seconds annealing at 60° C., 2 minutes extension at 72° C., followed by one cycle of 5 minutes at 72° C. The reaction components were: 1× cloned Pfu polymerase buffer (Stratagene), 100 μM each dNTP, 100 ng each primer, 10 ng template DNA, and 2Tl (20 units) of cloned Pfu DNA polymerase.

The products obtained from these two PCR amplifications were joined together in a third PCR amplification using the external primers PF3 [SEQ ID NO: 16] and PR5 [SEQ ID NO: 18]. This final product was gel purified, digested with BamHI and ligated into the corresponding site of vector pET11a.

The construct for expressing P33K-BACE was obtained by introducing the P33K mutation in pET11a-BACE by PCR using primers PF8 and PF9 as follows: [SEQ ID NO: 20] PF7 - 5′ CCGAGGAGAAAGGCCGGAGGG (forward) [SEQ ID NO: 21] PR8 - 5′ CCCTCCGGCCTTTCTCCTCGG (reverse) The codons for the substitution of Lysine for Proline at amino acid position 33 are underlined. Lysine is also coded by AAG. Accordingly the forward and reverse codons could also be AAG and CTT, respectively.

The products obtained from these two PCR amplifications were joined together in a third PCR amplification using the external primers for the pET11a vector: [SEQ ID NO. 22] PF9 -  5′ TAATACGACTCACTATAGG (forward,T7 promoter) [SEQ ID NO. 23] PF10 - 5′ GCTAGTTATTGCTCAGCGG (reverse,T7 termi- nator primer) This final product was gel purified, digested with BamHI and ligated into the corresponding site of vector pET11a. The complete DNA and amino acid sequence for the pET11a-P33K-BACE construct is shown in FIGS. 4A and 4B. The first fifteen amino acids (underlined) correspond to the vector's T7 tag and contain a BamHI cloning site as well as an additional methionine. Codon changes as preferred for expression in E. coli are shown in bold type.

Example 2 Cell Incubation and Inclusion Body Harvest

Ligated DNA was transformed into E. coli DH5α for propagation and DNA isolation. The resulting DNA was fully sequenced in both strands and then transformed into E. coli BL21 CodonPlus (DE3) Rp for expression. Cells were grown in Luria Broth (LB), pH 7.5, with 100 μg/ml ampicillin and 34 μg/ml chloramphenicol, at 37° C. and 200 rpm (2.5 inch throw). A loop of a glycerol stock of the construct was inoculated into the media and was incubated until the A₅₅₀=0.5-0.6. Cells were collected by centrifugation, resuspended in fresh media, and used as inoculum for a secondary culture at a 1:100 dilution. When cell density reached A₅₅₀=0.5-0.6, cells were harvested by centrifugation at room temperature and then resuspended at the same concentration in fresh LB, again containing ampicillin and chloramphenicol.

P33K-BACE expression was induced by the addition of IPTG to a final concentration of 1 mM. Expression of the recombinant protein was continued for 3 hours after induction (A₅₅₀=1.8-2.0). Cells were collected by centrifugation and stored at −80° C.

To determine the level of expression and localization of the recombinant protein, the collected centrifuged cells (cell paste) was resuspended in TE (10 mM Tris HCl pH8.0, 1 mM EDTA) at 1/10 of the original culture volume and sonicated. The soluble protein fraction was separated from cell debris and insoluble proteins by centrifugation at 10,000×g for 15 minutes. Protein in each of the fractions was analyzed by SDS-PAGE.

To obtain inclusion bodies, cultured cells were centrifuged to pellet the cells. Cell pellets were weighed from 1.0 liters of cell culture. The wet weight of the cell pellet was 2.25 g. The cell pellet was resuspended in 20 ml TE. The re-suspended cell pellet was subjected to 16,000 psi in a French press. The resulting solution was centrifuged at 6000 rpm for 30 minutes and then at 2900 rpm for 30 minutes in a Sorvall SS34 rotor. The pellet was then frozen at −20° C. for storage and later resuspended in 4 ml 8 M urea, 100 mM AMPSO, 1 mM glycine, 1 mM EDTA, and 100 mM BME, at pH 10.5-10.8. After centrifugation at 12,900 rpm in a Sorvall SS34 rotor for 40 minutes, the protein concentration of the supernatant was diluted 50 times with the above buffer (without BME) to read approximately 5.0-7.0 at A₂₈₀.

The P33K-BACE was refolded by diluting the resuspended protein 20-25 times with approximately 1700 ml of cold H₂O and adjusting the pH to 10.1 with a few drops of HCl. This dilution was stored for approximately 3 weeks at 4-15° C. prior to purification

Example 3 Purification of Refolded P33K-BACE Enzyme

A first purification step involved a Q-Sepharose™ FAST FLOW columns to concentrate the enzyme sample and remove nucleic acids present in abundance at this stage. The 1700 ml enzyme sample was loaded onto a 10 ml Q-Sepharose™ Fast Flow column was pre-equilibrated with 10 mM Tris (pH 8.2), 0.4 M urea and NaCl to bring the conductivity to 0.9 mMhos (to match the ionic strength of the BACE protein solution). A linear gradient of 0-1.0 M NaCl was applied in the same buffer used to equilibrate the column. Fractions of 5.5 ml were collected. Elutions of protein fractions were stored in a cold room.

A second purification step involved affinity purification. An affinity column was generated by coupling 1 mg of reduced Inhibitor I-1 (shown below) per ml of SulfoLink™ Coupling Gel (Pierce, Cat. No. 204011) in an Econo column (BioRad) in an amount of 1 mg/ml of the gel, according to the manufacturer's instructions.

The 36 ml BACE solution obtained from Q-Sepharose™ FAST FLOW purification was added to 9 ml of 1 M sodium acetate buffer, pH 4.5. No visible precipitate was observed. The sample was then loaded onto the affinity column pre-equilibrated with 200 mM sodium acetate buffer, pH 4.5. Elution from the affinity column was performed at pH 8.5 with 100 mM sodium borate. The eluted sample, 30 ml, was assayed for activity and analyzed on SDS-PAGE.

Example 4 Expression of BACE in CHO Cells (Control)

In order to provide a control for activity assays for P33K-BACE expressed in E. Coli and prepared as in Examples 1-3, a BACE construct (Asp2-2L-TM-His₆) encoding the amino acid sequence shown below in Table 2, was expressed in CHO cells and purified from about 75 liters of conditioned media. The purification process consisted of successive steps of tangential flow concentration, ammonium sulfate precipitation, Nickel affinity column, and affinity chromatography (I-1 affinity). However, the purified enzyme contains a 50:50 mixture of the isoforms starting at T¹QHGIRL . . . and E²⁵TDEEPEEPG . . . , numbered as in FIG. 1. The two isoforms are generated by post-translational cleavage by yet unknown proteases. It is readily expected that P33K-BACE can be expressed in CHO cells following site directed mutagenisis of the BACE polynucleotide. TABLE 1 BACE Construct Asp2-2L-TM-His₆ [SEQ ID NO: 24] MAQALPWLLL WMGAGVLPAH GTQHGIRLPLR SGLGGAPLG LRLPRETDEE PEEPGRRGSF VEMVDNLRGK SGQGYYVEMTV GSPPQTLNILV DTGSSNFA VGAAPHPFLH RYYQRQLSST YRDLRKGVYVP YTQGKWEGELG TDLVSIPH GPNVTVRANI AAITESDKFF INGSNWEGILG LAYAEIARPDD SLEPFFDS LVKQTHVPNL FSLQLCGAGF PLNQSEVLASV GGSMIIGGIDH SLYTGSLW YTPIRREWYY EVIIVRVEIN GQDLKMDCKEY NYDKSIVDSGT TNLRLPKK VFEAAVKSIK AASSTEKFPD GFWLGEQLVCW QAGTTPWNIFP VISLYLMG EVTNQSFRIT ILPQQYLRPV EDVATSQDDCY KFAISQSSTGT VMGAVIME GFYVVFDRAR KRIGFAVSAC HVHDEFRTAAV EGPFVTLDMED CGYNIPQT DESHHHHHH

Example 5 Activity Assay

The activity of P33K-BACE expressed in E. coli according the above Examples was compared to the activity of human BACE prepared as above without the P33K mutation, and to human BACE expressed in CHO cells. The results are provided in Table 2.

To assay for BACE activity, the following activity assay conditions were utilized, unless specifically indicated otherwise: 20 μl 1 M sodium acetate (NaOAc), pH 5.0; 125 μl H₂O; 50 μl BACE sample containing 5-10 pmoles protein; and 5 μl 1 mM substrate S-1 (SEQ ID NO: 10).

The 200 μl assay reaction was incubated for 1-2 hours at 37° C. Activity was expressed as fluorescence peak area generated by the liberation of a fluorescent (Tryptophan fluorescence) product upon cleavage of Substrate S-1 by recombinant BACE. The reaction was stopped with 100 μl of 4% TFA in H₂O. In order to separate the cleavage products and thereby detect activity of recombinant BACE, 50 microliters of the reaction mixture was injected into a Hewlett Packard Model 1100 HPLC equipped with an Alltech Rocket™ column (7 mm i.d., ×53 mm length, C₁₈, 3 μm) pre-equilibrated with 88% Reagent A (0.1% TFA in water), 12% Reagent B (0.1% TFA in acetonitrile). The flow rate over this column was 3 ml per minute. TABLE 2 Activity Assays (Fluorescence Peak Area) E. Coli P33K-BACE E. Coli BACE CHO BACE 38.41 31.03 83.32 26.50 45.80 85.20 32.84 31.86 78.07

The foregoing examples are illustrative of certain embodiments of the claimed invention, and do not serve to limit the invention in scope or spirit. 

1. An isolated polypeptide comprising human BACE having the modification Pro33Lys.
 2. The polypeptide of claim 1 comprising at least a portion of the transmembrane domain.
 3. The polypeptide of claim 1 comprising at least a portion of the C-terminal tail.
 4. The polypeptide of claim 1 comprising at least a portion of the signal peptide.
 5. A composition comprising an active human BACE enzyme comprising the pro-enzyme sequence of BACE having the modification Pro33Lys.
 6. The composition of claim 5, wherein the BACE comprises at least a portion of the transmembrane domain.
 7. The composition of claim 5, wherein the BACE, comprises at least a portion of the C-terminal tail.
 8. The composition of claim 5, wherein the BACE comprises at least a portion of the signal peptide.
 9. An isolated polypeptide comprising SEQ ID NO:
 2. 10. An isolated polypeptide consisting of SEQ ID NO:
 2. 11-21. (canceled) 